An iron core used in a transducer is made of amorphous thin strips, having an extremely thin thickness of 0.025 mm, which are layered to have a predetermined total thickness with a large number of the layered strips up to several hundreds or more. Thus, it is hard to transmit a heat to a center zone of the amorphous core. Further, heat-treatment conditions of the amorphous iron core are strict, so that the annealing furnace necessitates a higher temperature and a temperature control. It is also necessary to run an exciting current through the center zone of the iron core during annealing. By carrying out the above-mentioned measures, predetermined properties of the amorphous iron core can be obtained. Further, for the purpose of improving the heat treatment capacity, a plurality of iron cores are arranged in predetermined numbers of rows and stages whereby simultaneously subjecting a plurality of iron cores to the annealing treatment.
A conventional annealing furnace is filled with inert gas in order to prevent the iron core from oxidizing and transmit a heat to the iron core through the inert gas. The furnace is so structured to have a heater portion, a circulation fun, and a cooling portion in the furnace. In the furnace, a temperature of the gas is controlled in the heater portion and the cooling portion, and it is circulated in the furnace by means of a circulation fan. There are two ways of gas circulation in the furnace, which are a transversely feeding way and a vertically feeding way.
Further, there are also two ways of feeding the inert gas, according to a first way of which the inert gas is continuously fed into the furnace at a predetermined rate, and a second way of which the furnace is evacuated, and subsequently the inert gas is filled in the furnace.
The temperature of the inert gas is controlled by means of a temperature control unit which is commercially available, in a temperature pattern which is classified heating, maintaining homogenous heat, and cooling.
Further, JP-A-5-18682 discloses a method adapted to aim at rectifying a hot gas so as to homogenize the distribution of temperature during circulation of the hot gas, but a temperature difference is inevitably caused among an inlet, an outlet for the hot gas, and a center position of the furnace.